Starting means for a single-phase asynchronous motor

ABSTRACT

The invention relates to a starting switch assembly for a single phase asynchronous motor. The assembly includes a symmetrically controlled semiconductor valve in series with the motor starting coil. The control electrode of the valve is connected by way of a PTC resistor to the end of the starting coil which is remote from the valve. In this arrangement the PTC resistor is controlled directly by the power supply voltage and the time required for the PTC resistor to reach the cutout temperature depends substantially only on the characteristics of the PTC resistor and is independent of the particular construction of the motor.

United States Patent Inventor Asger Gramkow Gammelgard pr. Augustenborg,Denmark Appl. No. 7,890 Filed Feb. 2, 1970 Patented Aug. 17, 1971Assignee Danfoss A/S Nordborg, Denmark Priority Mar. 25, 1969 Germany P19 15 135.0

STARTING MEANS FOR A SINGLE-PHASE References Cited Primary ExaminerGeneZ. Rubinson Attorney-Wayne B. Easton ABSTRACT: The invention relates toa starting switch assembly for a single phase asynchronous motor. Theassembly includes a symmetrically controlled semiconductor valve inseries with the motor starting coil. The control electrode of the valveis connected by way of a PTC resistor to the end of the starting coilwhich is remote from the valve. In this arrangement the PTC resistor iscontrolled directly by the power supply voltage and the time requiredfor the PTC resistor to reach the cutout temperaturedepends'substantially only on the characteristics of the PTC resistorand is independent of the particular construction of the motor.

ASYNCHRONOUS MOTOR 3 Claims, 1 Drawing Fig. 03. Cl 318/221 E, 318/227,318/229 Int. Cl H02p 5/40, H02p 1/44 Field of Search 318/220, i 221,227, 229

STARTING MEANS FOR A SINGLE-PHASE ASYNCHRONOUS MOTOR The inventionrelates to a starting switch for a single-phase asynchronous motor inthe form of a symmetrically controlled semiconductor valve, arranged inseries with the starting coil.

A starting switch of this kind is known in which the semiconductor valveis controlled with the help of a transformer in dependence upon thecurrent passing through the main coil. Here, the primary winding of thetransformer is arranged in series with the main coil, whereas'thesecondary winding connects a contact on the valve to its controlelectrode. A transformer of this kind is however too complicated,expensive and bulky for many applications.

It is also known to use a FTC-resistor for starting a singlephaseasynchronous motor. For example, the FTC-resistor is connected in serieswith the starting coil and is heated by the current that flows throughand its resistance is raised, so that the starting current is reduced toan insignificant value in a prescribed period. In this arrangement, theFTC-resistor must initially carry the full starting current.'This leadsto a thermal shock which FTC-resistors, usually formed as ceramicelements, cannot often withstand.

In accordance with an earlier proposal, the semiconductor valve isconnected in series with an ohmic resistor in the starting coil, and thecontrol electrode thereof is connected through a FTC-resistor to thatend of the valve remote from the ohmic resistor. In the case of thisinherently very useful switch it is however necessary for the ohmicresistor and the FTC-resistor to be precisely rated tosuit the data ofthe asynchronous motor, since the voltage-drop at the ohmic resistor,used for control purposes, is dependent upon the magnitude of thestarting current. The object of the invention is to provide asemiconductor starting switch which can be operated using extremelysimple means and which can be used for a large number of differentmotors.

According to the invention, this object is achieved by connecting thecontrol electrode of the semiconductor valve to that end of the startingcoil remote from the valve, by way of a FTC-resistor.

In this arrangement, the heating-up of the FTC-resistor is controlled bythe constant mains voltage. The period required by the FTC-resistor toreach that temperature at which the control voltage blocks thesemiconductor valve depends substantially only upon the characteristicsof the FTC-resistor and is independent of the particular construction ofthe motor.

If the current flowing across the control gap of the semiconductor valveis not sufficient to hold the FTC-resistor at a sufficiently high levelof resistance, i.e., at a sufficiently high temhas to provide systemsoperating at different powers, but, for reasons of rationalization triesto make do with one or just a few types of starting switch.

Here, in particular, the semiconductor valve, the PTC-resistor, themotor and, where required, the shunt resistor can be accommodated insidethe case of a small refrigerating machine, the case being holed fortwo-pole connection only.

The invention will now be described in more detail by reference to thedrawing which shows a connection diagram for the starting switch of theinvention.

A single-phase motor 1, having a main coil 2 and a starting coil 3, isconnected to the terminals 5 and 6 of the supply system through a mainswitch 4. Arranged in series with the starting coil 3 is a symmetricalsemiconductor valve 7, for example a multilayer diode, marketed underthe trade name Triac. The control electrode 8 of this valve is connectedby way of a FTC-resistor 9 to a point 10 on that end of the startingcoil 3 remote from the valve 7.

When the main switch 4 is switched on, the FTC-resistor 9 is at firstcold. There is thus applied to the control electrode 8 a voltage whichis only slightly below the voltage at the supply terminal 6.Consequently the semiconductor valve 7 strikes and the motor starts torun. Because of the flow of current, induced by the supply voltage, theFTC-resistor 9 heats up until it reaches so great a resistance that thevoltage available at the control electrode 8 no longer suffices tostrike the valve 7, so that the starting coil 3 is cut out. There is nodifficulty in so rating the FTC-resistor 9 that the moment of cutoutoccurs when it is certain that the motor 1 is running at speed.

If required, a shunt resistor 11, bypassing the control gap of thesemiconductor valve 7, can be provided. This, together with theFTC-resistor 9, constitutes a voltage divider at the tapping point 12 ofwhich the control voltage can be tapped off.

i The FTC-resistor 9 also prevents any attempt at repeatedly switchingon again, since then the FTC-resistor will not have cooled downsufficiently during the starting operation to make a control voltagesufficient for striking available.

The parts 1, 7, 9 and, possibly, 11 are accommodated within a case 13containing only two connection orifices 14. This results in a muchsimpler construction for an encased refrigerating machine.

The expression symmetrical, controlled semiconductor valve is intendedto cover such control elements that permit the current to pass in bothdirections when an adequate striking signal exists.

I claim:

1. A motor and control assembly comprising a single-phase asynchronousmotor having a main coil and a starting coil in parallel with said maincoil, said coils having leads extending to a junction, a symmetricalcontrolled semiconductor valve in series with said starting coil on theopposite side of said starting coil from said junction, said valvehaving a control electrode, and a FTC-resistor between said controlelectrode and said junction.

2. An assembly according to claim 1 including a shunt resistor betweensaid control electrode and the side of said valve remote from saidstarting winding.

3. An assembly according to claim 1, a refrigeration machine casing,said assembly being enclosed in said casing, and terminal meansinternally of said casing for the attachment thereto of power supplyconductors.

1. A motor and control assembly comprising a single-phase asynchronousmotor having a main coil and a starting coil in parallel with said maincoil, said coils having leads extending to a junction, a symmetricalcontrolled semiconductor valve in series with said starting coil on theopposite side of said starting coil from said junction, said valvehaving a control electrode, and a PTC-resistor between said controlelectrode and said junction.
 2. An assembly according to claim 1including a shunt resistor between said control electrode and the sideof said valve remote from said starting winding.
 3. An assemblyaccording to claim 1, a refrigeration machine casing, said assemblybeing enclosed in said casing, and terminal means internally of saidcasing for the attachment thereto of power supply conductors.